Add warning log when metadata query hits 10000 result limit (#14109)

## What problem does this PR solve?

Add a warning log when `get_flatted_meta_by_kbs` returns 10,000 results,
which indicates the query limit has been reached and metadata may be
silently truncated.


## Type of change
- [x] Improvement (non-breaking change which improves observability)

api/db/services/doc_metadata_service.py

@@ -733,9 +733,11 @@ class DocMetadataService:
 
             # Aggregate metadata
             meta = {}
+            doc_count = 0
 
             # Use helper to iterate over results in any format
             for doc_id, doc in cls._iter_search_results(results):
+                doc_count += 1
                 # Extract metadata fields (exclude system fields)
                 doc_meta = cls._extract_metadata(doc)
 
@@ -752,6 +754,9 @@ class DocMetadataService:
                             meta[k][sv] = []
                         meta[k][sv].append(doc_id)
 
+            if doc_count >= 10000:
+                logging.warning(f"[get_flatted_meta_by_kbs] Results hit the 10000 limit for KBs {kb_ids}.")
+
             logging.debug(f"[get_flatted_meta_by_kbs] KBs: {kb_ids}, Returning metadata: {meta}")
             return meta